Inhibition of silica and silicate deposition using ethylene oxide-propylene oxide block copolymers

ABSTRACT

A method of controlling the deposition of silica-containing scales and their adhesion to surfaces in contact with an aqueous system is disclosed which comprises adding to the aqueous system an ethylene oxide-propylene oxide copolymer.

FIELD OF THE INVENTION

The present invention relates to controlling silica deposits in aqueoussystems. More particularly, the present invention relates to inhibitingsilica/silicate deposition in cooling and boiler water systems usingethylene oxide-propylene oxide block copolymer.

BACKGROUND OF THE INVENTION

The problem of scale formation and its attendant affects have troubledwater systems for many years. For instance, scale tends to accumulationthe internal walls of various water systems, such as boiler and coolingwater systems, thereby reducing heat transfer properties and fluid flowthrough heat exchanger tubes.

One particular type of deposit, silica, is especially troublesome insome systems. Where the water used in cooling the systems and watercooled industrial heat exchangers is taken directly from lakes, rivers,ponds, or municipal water sources, various amounts of dissolved andsuspended solids, including silica, are present. Problems are compoundedin operations where water is concentrated, or cycled up, during theprocess, e.g., cooling systems. As the water evaporates, the silicaconcentrations increase increasing both the occurrence and the degree ofdeposition. Increasing silica concentrations can also result inmonomeric silica undergoing polymerization to form gelular and/orcolloidal silica which can form tenacious deposits.

In cooling water systems, silica and silicate compounds form deposits onthe internal metal surfaces in contact with the water flowing throughthe system. In this manner, heat transfer efficiency becomes severelyimpeded, which in turn has a deleterious effect on the overall operatingefficiency of the cooling water system. Silica and silicate depositionalso causes problems on other conduit and piping surfaces, as well as onequipment such as valves, nozzles and pumps.

Although current industrial cooling systems make use of sophisticatedexternal treatments of the feed water, e.g., coagulation, filtration,softening of water prior to it being fed into the water system, theseoperations are only moderately effective. In all cases, externaltreatment does not in itself provide adequate treatment since muds,sludge, silts, and dissolved solids such as silicate can escape thetreatment and eventually are introduced into the system.

Silica (silicon dioxide) appears naturally in a number of crystallineand amorphous forms, all of which are sparingly soluble in water; thusleading to the formation of undesirable deposits. Silicates are saltsderived from silica or the silicic acids, especially orthosilicates andmetosilicates, which may combine to form polysilicates. All of these,except the alkali silicates are sparingly soluble in water. A number ofdifferent forms of silica and silicate salt deposits are possible, andformation depends, among other factors, on the temperature and pH of thewater.

Various methods have been utilized for resolving the problem of sludgeand silt, including silica, deposition. U.S. Pat. No. 5,378,368discloses the use of polyether polyamino methylene phosphonates tocontrol silica/silicate deposition in industrial water systems. Thepolyether polyamino methylene phosphonates may be used alone or incombination with a polymer additive.

U.S. Pat. No. 5,078,879 discloses the use of 2-phosphono-butanetricarboxylicate acid-1,2,4 alone or preferably in combination with ananionic polymer such as a carboxylic/sulfonic polymer, to control theformation of silica/silicate deposits in aqueous systems.

U.S. Pat. No. 4,933,090 discloses the use of a select phosphonate andoptionally a carboxylic/sulphonic/polyalkylene oxide polymer to controlsilica/silicate deposition.

U.S. Pat. No. 4,874,527 discloses the use of an imine polymer, aphosphanate and optionally a source of molybdate or borate ions tocontrol the formation of silica/silicate deposits in aqueous systems.

U.S. Pat. No. 5,158,685 discloses the use of a combination of ahydroxyphosphonoacetic acid and an acrylic acid/alyl hydroxy propylsulphonate ether polymer to control silica/silicate deposition incooling water systems.

U.S. Pat. No. 5,300,231 discloses the use of polyether polyaminomethylene phosphonates in combination with hydroxy-phosphono acetic acidor amino tris methylene phosphonate to control silica/silicatedeposition in various industrial water systems.

U.S. Pat. No. 4,405,461 discloses the use of a treatment comprising anamine to which is attached at least a pair of terminal groups selectedfrom furfuryl and saturated or unsaturated hydrocarbon radicalssubstituted with one or more hydroxy and carboxy, or a hydrohalidethereof to control the deposition of silica-containing scales.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, it has been discovered thatethylene oxide-propylene oxide block copolymers thereof are effectivetreatment agents for reducing the deposition of silica/silicate inaqueous systems. The method of the present invention comprises adding aneffective amount of an ethylene oxide-propylene oxide block copolymer ormixtures thereof to an aqueous system being treated.

The treatment for aqueous systems of the present invention is acopolymer of ethylene oxide and propylene oxide (EO-PO). The preferredcopolymer has an average molecular weight ranging from about 1000-5000.The molecular weight of the hydrophobe (polyoxypropylene) preferablyranges from about 950-3000, and the percent of hydrophile(polyoxyethylene) ranges from about 20-70% of the copolymer. EO-POcopolymers are available commercially from BASF under the Pluronic™label.

An effective amount of EO-PO copolymer can be added to an aqueous systembeing treated. As used herein, the term effective amount is that amountnecessary to control silica/silicate deposition in the system beingtreated. Generally, the effective amount will range from about 1 to 100ppm, on an active basis based upon the total weight of the aqueoussystem being treated.

As used herein, the term controlling the silica/silicate deposition isto include inhibition of silica polymerization, threshold precipitationinhibition, stabilization, dispersion, solubilization, and/or particlesize reduction of silica, silicates, calcium and magnesium silicates,and silicon ions. The treatments of the present invention are thresholdsilicate precipitation inhibitors which also stabilize, disperse andsolubilize silica and silicates, and generally reduce the particle sizeof any precipitated material.

Aqueous system as used herein, is meant to include any type of systemcontaining water, including, but not limited to, cooling water systems,boiler water systems, desalination system, gas scrubber water systems,evaporator systems, paper manufacturing systems, mining systems, and thelike.

The EO-PO copolymers of the present invention are well known to thoseskilled in water treatment art, and are commercially available. Thetreatment materials of the present invention may be added to the aqueoussystem being treated by any convenient means. A preferred method ofaddition is to the makeup water streams. Additionally, otherconventional water treatment agents such as corrosion inhibitors can beused in combination with treatments of the present invention.

The present invention will now be further described with reference to anumber of specific examples which are to be regarded as illustrative,and not as restricting the scope of the present invention.

EXAMPLES

The efficiency of EO-PO copolymer chemistry to inhibit silica/silicatedeposition in an aqueous system was evaluated in a stirred batch reverseosmosis apparatus. The apparatus was a pressurized water filter wherethe filter media was a reverse osmosis membrane and the pressure wasprovided by compressed nitrogen. An overhead stirrer paddle waspositioned near the surface of the membrane filter to provide turbulenceto minimize concentration effects.

A synthetic feed water containing 100 ppm silica as SiO₂ was added tothe apparatus with and without treatment. The feed water wasconcentrated approximately ten times over a 30-40 minute period, suchthat the water at the end of the experiment contained about 1000 ppmSiO₂. At the conclusion of a run, the reverse osmosis membrane wasremoved and analyzed for silica to quantify the amount of silicadeposited on the membrane. Tests with a chemical treatment were comparedto control tests (no chemical treatment) to calculate a percent silicainhibition. The results are summarized in Table I.

                  TABLE I                                                         ______________________________________                                                    Average                Percent                                       Molecular Percent Silica                                                     Treatment Weight Polyoxyethylene Inhibition                                 ______________________________________                                        10 ppm Pluronic L35                                                                       1900      50           27                                           10 ppm Pluronic L42 1630 20 33                                                10 ppm Pluronic L43 1850 30 41                                                10 ppm Pluronic L44 2200 40 45                                                10 ppm Pluronic L61 2000 10 29                                                10 ppm Pluronic L62 2500 20 34                                                10 ppm Pluronic L63 2650 30 37                                                10 ppm Pluronic L64 2900 40 28                                                10 ppm Pluronic P65 3400 50 27                                                10 ppm Pluronic P68 8400 80  0                                                10 ppm Pluronic P84 4200 40 29                                                10 ppm Pluronic L92 3650 20 24                                                10 ppm Pluronic F108 14600  80  0                                             10 ppm Pluronic L121 4400 10  0                                             ______________________________________                                    

The data in Table I shows that EO-PO copolymers comprising from about20-70% hydrophile (polyoxyethylene) and an average molecular weight fromabout 1000-5000 provide significant silica inhibition in aqueoussystems. The EO-PO copolymer treatment of the present invention iseffective at inhibiting silica deposition in systems having relativelylow (<180 ppm) concentrations of reactive silica.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of this invention will be obvious to those skilled in theart. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true spirit and scope of the present invention.

What is claimed is:
 1. A method of inhibiting the deposition of silicaand silicate compounds on surfaces in contact with an aqueous systemconsisting essentially of adding to the aqueous system an effectiveamount to inhibit deposition of silica and silicate compounds ofethylene oxide-propylene oxide copolymer of average molecular weightfrom 1000-5000 having a percent hydrophile ranging from about 20-70%. 2.The method of claim 1 wherein said ethylene oxide-propylene oxidecopolymer is added at a dosage of about 1 to 100 ppm.
 3. A method ofsolubilizing silica and silicate compounds in aqueous systems consistingessentially of adding to the aqueous system an effective amount tosolubilize silica and silicate compounds of ethylene oxide-propyleneoxide copolymer of average molecular weight from 1000-5000 having apercent hydrophile ranging from about 20-70%.
 4. The method of claim 3wherein said ethylene oxide-propylene oxide copolymer is added at adosage of from about 1 ppm to about 100 ppm.
 5. A method of dispersingsilica and silicate compounds in aqueous systems consisting essentiallyof adding an effective amount to disperse silica and silicate compoundsof ethylene oxide-propylene oxide copolymer of average molecular weightfrom 1000-5000 having a percent hydrophile ranging from about 20-70%. 6.The method of claim 5 wherein said ethylene oxide-propylene oxidecopolymer is added at a dosage of from about 1 ppm to about 100 ppm.